Generally, in a liquid crystal display, it is required to dispose a polarizer on both sides of a glass substrate of a liquid crystal panel in order to provide bright images with good color saturation. The polarizer is generally manufactured by dyeing a polyvinyl alcohol (PVA)-based film with a dichroic dye such as iodine, crosslinking the dyed film using a crosslinking agent, and then aligning the molecules of the film by a process such as uniaxial stretching. Because the polarizer is manufactured by stretching, it is easy to shrink. Particularly, because the polyvinyl alcohol-based film is based on a hydrophilic polymer, it is easy to deform under moist heat conditions. In addition, because the film itself has low mechanical strength, problems such as tearing of the film may arise. For these reasons, a polarizing plate is used, which is obtained by bonding a protective film to one or both sides of the polarizer to enhance the strength.
Meanwhile, in recent years, the application of liquid crystal displays has been expanded to include mobile terminals, and wide-screen televisions for home use. Thus, technology has been developed in order to guarantee excellent display qualities in each liquid crystal display. In the display qualities of liquid crystal displays, the colors of a polarizer and a polarizing plate are as important as the degree of polarization.
In the prior art, in order to control the color of polarizing plates, methods have been used, including controlling the time during which a polyvinyl alcohol-based film is immersed in a treatment bath, or controlling the temperature of the treatment bath.
However, the above-described methods of controlling the time of immersion of the film in the treatment bath and the temperature of the treatment bath have a problem in that, when stretching conditions in a stretching process are changed, it is difficult to control the color, because the time of immersion of the film in the treatment bath and the temperature of the treatment bath should also be controlled.
For this reason, a color complement process of immersing a PVA-based film stretched in an aqueous potassium iodide (KI) solution was added during the manufacture of PVA-based polarizers in the prior art. Among the polarizer manufacturing processes, the color complement process employing the aqueous KI solution corresponds to a process that increases again the absorbance of the polarizer in a short-wavelength range (400-500 nm), which was reduced after the stretching process, thereby controlling the electro-optic characteristics of the polarizing plate, particularly the color.
However, in the above-described color complement process employing KI, when KI is excessively used to increase the color complementary effect, the amount of KI remaining in the polarizer manufactured by the above-described process will increase, thereby increasing the occurrence of cunic defects (local surface irregularities), glittering, the generation of foreign matter defects in the polarizing plate, etc. In addition, when a polarizing plate is exposed to a high temperature higher than 80° C. for a long period of time, the degree of polarization of the polarizing plate will decrease, and the color will change, indicating that the polarizing plate has poor heat resistance.
As described above, the prior art method for controlling the color of a polarizing plate has problems in that it is difficult to control the color and in that the physical properties of the manufactured polarizing plate are also reduced. For this reason, there is a need to develop a new method for controlling the color of a polarizing plate.